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62 lines
3.0 KiB
62 lines
3.0 KiB
2 months ago
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=========================================
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strcmp() / memcmp() token capture library
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=========================================
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(See ../docs/README for the general instruction manual.)
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This Linux-only companion library allows you to instrument strcmp(), memcmp(),
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and related functions to automatically extract syntax tokens passed to any of
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these libcalls. The resulting list of tokens may be then given as a starting
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dictionary to afl-fuzz (the -x option) to improve coverage on subsequent
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fuzzing runs.
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This may help improving coverage in some targets, and do precisely nothing in
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others. In some cases, it may even make things worse: if libtokencap picks up
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syntax tokens that are not used to process the input data, but that are a part
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of - say - parsing a config file... well, you're going to end up wasting a lot
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of CPU time on trying them out in the input stream. In other words, use this
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feature with care. Manually screening the resulting dictionary is almost
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always a necessity.
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As for the actual operation: the library stores tokens, without any deduping,
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by appending them to a file specified via AFL_TOKEN_FILE. If the variable is not
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set, the tool uses stderr (which is probably not what you want).
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Similarly to afl-tmin, the library is not "proprietary" and can be used with
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other fuzzers or testing tools without the need for any code tweaks. It does not
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require AFL-instrumented binaries to work.
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To use the library, you *need* to make sure that your fuzzing target is compiled
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with -fno-builtin and is linked dynamically. If you wish to automate the first
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part without mucking with CFLAGS in Makefiles, you can set AFL_NO_BUILTIN=1
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when using afl-gcc. This setting specifically adds the following flags:
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-fno-builtin-strcmp -fno-builtin-strncmp -fno-builtin-strcasecmp
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-fno-builtin-strcasencmp -fno-builtin-memcmp -fno-builtin-strstr
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-fno-builtin-strcasestr
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The next step is simply loading this library via LD_PRELOAD. The optimal usage
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pattern is to allow afl-fuzz to fuzz normally for a while and build up a corpus,
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and then fire off the target binary, with libtokencap.so loaded, on every file
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found by AFL in that earlier run. This demonstrates the basic principle:
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export AFL_TOKEN_FILE=$PWD/temp_output.txt
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for i in <out_dir>/queue/id*; do
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LD_PRELOAD=/path/to/libtokencap.so \
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/path/to/target/program [...params, including $i...]
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done
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sort -u temp_output.txt >afl_dictionary.txt
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If you don't get any results, the target library is probably not using strcmp()
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and memcmp() to parse input; or you haven't compiled it with -fno-builtin; or
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the whole thing isn't dynamically linked, and LD_PRELOAD is having no effect.
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PS. The library is Linux-only because there is probably no particularly portable
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and non-invasive way to distinguish between read-only and read-write memory
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mappings. The __tokencap_load_mappings() function is the only thing that would
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need to be changed for other OSes. Porting to platforms with /proc/<pid>/maps
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(e.g., FreeBSD) should be trivial.
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